Plate Tectonics

Slow alteration of gene pool by changing landscape over Geological Time.

In the oceans, species don't have to deal with whole changes in the physical characteristics of their ecosystem.

The evolution of species on the land is linked to and driven by various climatological and geological changes that operated on the land surface of the earth.

As we will discuss later, the earth currently has significant climate variations on a timescale of 100,000 years. In addition, over the last 200-250 million years the earth is experiencing an era go global tectonic motion which makes the land surface a Dangerous Place to Live:

• Map of recent earthquakes Western USA
• Map of recent earthquakes World
• Map of Current LA Earthquakes

Plate Tectonics means that the crust of the earth is divided into large connected units, all of which are moving relative to one another and colliding with one another in various ways. The idea of Plate Tectonics was first published by the German geologist, Alfred Wegener in 1915 but this theory was largely ridiculed until magnetic mapping of the ocean floor was done in the late 50's. This history of the acceptance of this theory is summarized here .

Summary of Evidence for Plate Tectonics:

• The continents appear that they could fit together like a Jigsaw Puzzle. Just a coincidence?

• Similar fossil records are found in Brazil and North Africa could just mean that the climates and hence vegetation and animal life were similar

• Similar rock types are seen in Brazil and North Africa this is much harder to explain away especially if the ages of the rock types are similar.

• Sea Floor Spreading Magnetic mapping of the ocean floor revealed a history of polarity reversals that formed a mirror image about the Mid-Atlantic Ridge. This indicates that the floor of the Atlantic Ocean is created at the mid Atlantic ridge due to crustal separation. As magma issues forth and solidifies, it records the orientation of the earth's magnetic field at the time of solidification. These newly formed rocks are then carried either East or West of the Mid-Ocean ridge. The floor of the Atlantic Ocean is thus an Age sequence with newly formed rock appearing at the Mid Atlantic Ridge and the oldest rocks being at the North American and European Coastlines. The time of transport from the mid Atlantic ridge to these coastlines is about 210 million years which indicates a total sea floor motion of about 1.5 inches per year.

Now it is recognized that the surface of the earth can be divided up into roughly 10--12 large scale plates and perhaps a number of smaller ones as in the case of the Pacific Northwest

Plate Tectonics Continued


Break up of Pangea

---

The driving mechanism of plate tectonics is a network of convective heat currents, generated by the hot core of the earth and which circulate in the mantle. The heat is provided from the decay of Uranium-238 which is an R-process Supernova element. The overall transport of heat from the core through the mantle is quite inefficient so it takes a long time for these convective heat currents to become established. Hence, plate movements are something which occurs late (i.e. now) in the geological history of the earth.

The earth's crust is actually a two-component layer. The lithosphere is a thin layer of rock (average density of 2.7 grams per cc) and "floats" on top of a plastic-like layer called the asthenosphere. Plastic-like materials are weird - they deform under stress but don't really break. A glacier is a good example of a material that moves and flows plastically. The convective heat currents in the mantle impinge on the asthenosphere causing deformation and subsequent movement of the lithospheric plates.

This process can be simulated in your kitchen by putting some jello in a bowl and putting some peebles on top of the jello. As you shake the bottom of the bowl, the jello deforms but doesn't break and the rocks that float on the jello collide. (apologies to real geologists for this analogy).

As a result of plate movements, interesting things occur at plate boundaries. In general you don't want to live near a plate boundary as the earth is active there. About 75% of the world's population does live near these boundaries.

There are three types of plate boundaries:

• A divergence zone crustal separation two plates are moving apart in opposite directions
• A convergence zone collision of two plates. A collision of a less dense continental plate with a more dense oceanic plate creates a subduction zone where the denser plate dives (subducts) beneath the less dense plate. A collision between two continental plates results in general uplift.
• A translational zone here two plates slide by one another in opposite directions. The San Andreas Fault is the most well-known (and potentially most deadly) translational interface.

Local Manifestations of Plate Tectonics:

• Tectonic Details
• Pacific Northwest Details
• Find the Fault lines
• Mt. Adams
• Mt. Rainier
• Mt. Lassen
• Mt. St. Helens

The Pacific Northewest is an active tectonic zone. One of the prime hazards of active volcanoes is the heavy mudflows which can result from the sudden melting of their heavily glaciated slopes. Some examples of Northwest Hazards are shown below:

• Volcanoes all in a Line
• Recent Lava Intrusions
• Basalt Flows in the Pacific Northwest
• Mudflows from Mt. Rainier

Some Other Internet Resources on Plate Tectonics:

Another overview of all of this

Imaging the Ocean Floor

Previous Lecture Next Lecture Course Page